IPES was performed at room temperature using the experimental geometry of Fig. REF (b) with the GM1 counter. The GaAs photocathode was illuminated with IR light of positive and negative helicity to invert the spin polarization of photoemitted electrons. Raw spectra are summarized in Fig. REF (c)-(d). Fig. REF (c) shows the SS dispersion while Fig. REF (d) shows the surface resonance (SR) and the bulk states labeled as B1 and B2.
The spin-split SS cuts the Fermi edge at \(k^{\downarrow }_{F} \sim 0.153\)  Å\(^{-1}\)  and \(k^{\uparrow }_{F} \sim 0.177\)  Å\(^{-1}\)  for the spin down and spin up components, respectively[1]}. This corresponds to incidence angles of \(\theta ^{\downarrow }_{F}\sim 8\)  and \(\theta ^{\uparrow }_{F}\sim 9\)  for the photon energy threshold of the GM1 counter. The SS increases intensity when approaching \(k_{F}\)  from lower angles. Conversely, the SR band disperses towards lower energies and its signal decreases in intensity until it is no longer detectable at \(\theta >38\) . Similarly, B1 disperses to lower energies but with increasing signal intensity and with a clear spin asymmetry (\(\theta \ge 48\) ) [2]}. From Fig. REF  it can be observed an energy spin splitting of \(\Delta E \sim 140\pm 20\)  meV in agreement with the expected experimental splitting (110 meV) and theoretical estimation (\(\sim \) 150 meV) [1]}. The individual fitting of the observed peaks in Fig. REF , by Voigt functions over linear backgrounds, allows determining the \(E(\mathbf {k_{||}})\)  relation (Fig. REF ). The SS follows the nearly-free-electron model [4]} and becomes a SR in the projected bulk band region.
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